diff --git a/core/lib/Drupal/Core/Entity/Schema/ContentEntitySchemaHandler.php b/core/lib/Drupal/Core/Entity/Schema/ContentEntitySchemaHandler.php
index 51e7a67..d0a0b1a 100644
--- a/core/lib/Drupal/Core/Entity/Schema/ContentEntitySchemaHandler.php
+++ b/core/lib/Drupal/Core/Entity/Schema/ContentEntitySchemaHandler.php
@@ -158,7 +158,11 @@ protected function addFieldSchema(array &$schema, $field_name, array $column_map
       // table. For this reason the revision ID field cannot be marked as NOT
       // NULL.
       unset($keys['label'], $keys['revision']);
-      $schema['fields'][$schema_field_name]['not null'] = in_array($field_name, $keys);
+      // Key fields may not be NULL.
+      $is_required = in_array($field_name, $keys);
+      if ($is_required) {
+        $schema['fields'][$schema_field_name]['not null'] = $is_required;
+      }
     }
 
     if (!empty($field_schema['indexes'])) {